Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The charge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be implemented using a number of different architectures. For example, in a single pass printer a composite final image is produced in one pass of the photoreceptor through the machine. A second architecture is a four pass printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine and wherein the composite color image is transferred and fused during the fourth pass. REaD IOI can also be implemented in a five cycle printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine, but wherein the composite color image is transferred and fused during a fifth pass through the machine.
The single pass architecture is very fast, but expensive since four charging stations and four exposure stations are required. The four pass architecture is slower, since four passes of the photoreceptive surface are required, but also much cheaper since it only requires a single charging station and a single exposure station. Five cycle printing is even slower since five passes of the photoreceptive surface are required, but has the advantage that multiple uses can be made of various stations (such as using a charging station for transfer). Furthermore, five cycle printing also has the advantage of a smaller footprint. Finally, five cycle printing has a decided advantage in that no color image is produced in the same cycle as transfer, fusing, and cleaning when mechanical loads are placed on the drive system.
The residual image phenomenon is observed as a faint image of a previous document in initial copies of a new document after the previous document has been repeatedly imaged on the photoreceptor, i.e., after the photoreceptor has been cyclically charged overall and discharged, repeatedly in registry, by the light pattern from the previous document. This residual image effect is believed to be caused by the accumulation of charges trapped within the charge generating layer of the photoreceptor in an imagewise pattern corresponding to the previous document image. The speed (rate of discharge per unit exposure) of the photoreceptor is modified by this accumulation of trapped charges so that, upon exposure to a new document, the areas of the photoreceptor associated with the previous document pattern are discharged proportionally to their previous history and the new image is developed with toner simultaneously with a ghost of the previous image. It will be readily appreciated that such a ghost image is detractive from the esthetic viewpoint; however, the provision of previous document information in the subsequent document prints presents an even more serious problem when proprietary information is embodied in the previous document.
It is well known that fatigue of the type causing the residual image effect in photoconductive insulator members can be relieved to some extent by application of infrared radiation to, or otherwise heating, such members or by an overall flooding of such members with light (see for example U.S. Pat. No. 2,863,767). Also, it has been noted that some regeneration of such a fatigued member can be effected by application of an electrostatic charge, of polarity opposite that of the primary (sensitizing) charge, at some time after the development step and before any subsequent sensitizing step of a copy/print cycle (see for example U.S. Pat. No. 2,741,959). However, in certain electrophotographic apparatus, e.g., one employing a REaD IOI process, in which a photoreceptor is rapidly exposed a large number of times to the same image, and in which the latent image is not completely erased between each subsequent exposure and development step, the residual image problem is more pronounced. Specifically, in the ReaD IOI process, the differential history of each portion of the image area, with parts being charged and recharged at each subsequent station without exposure while others are charged and exposed several times, causes a pronounced residual image problem. In this case, the above-noted prior art techniques have been found impractical and/or to inadequately eliminate residual image, at least in certain such members.
To erase residual electrostatic charge from the photoreceptor, conventional printing machines employ an erase source that either faces the image area on the front surface of the photoreceptor ("front erase") or faces and penetrates semi transparent or translucent layers from the rear of the photoreceptor ("rear erase"). This conventional arrangement generally has been adequate for black and white reproductions and in color machines employing three or more pass architectures. The present inventors, however, have determined that conventional erase arrangements may be inadequate in certain situations for high quality color reproductions and especially for printing machines employing a single pass image on image architecture (with no erase after every development station). Conventional erase arrangements may create ghost images (i.e., residual image effect) and slight voltage non-uniformities that result in objectionable color shifts. Thus, there is a need, which the present invention addresses for new apparatus and new methods that can alleviate the above described residual image problem.
Electrostatic charge erase apparatus and methods, as well as other parts of printing machines, are disclosed in Staudenmayer et al., U.S. Pat. No. 4,035,750; Castelli et al., U.S. Pat. No. 5,748,221; Folkins et al., U.S. Pat. No. 5,848,335; Kaukeinen et al., U.S. Pat. No. 5,394,230; and Nakashima et al., U.S. Pat. No. 4,728,985; Tabb et al., U.S. Pat. No. 5,778,288; Facci et al., U.S. Pat. No. 5,079,121 (the use of a tungsten erase lamp is disclosed in column 21, line 24); and Pollutro et al., U.S. Pat. No. 5,933,177 (discloses the use of an ion stream to eliminate surface charge).